ATmega88PA Automotive Atmel Corporation, ATmega88PA Automotive Datasheet - Page 94

• Manufacturer Part Number: ATmega88PA Automotive
• Manufacturer: Atmel Corporation

15.2.1
15.2.2
15.3
15.4
94
Timer/Counter Clock Sources
Counter Unit
Atmel ATmega48PA/88PA/168PA [Preliminary]
Definitions
Registers
Many register and bit references in this section are written in general form. A lower case “n”
replaces the Timer/Counter number, in this case 0. A lower case “x” replaces the Output Com-
pare Unit, in this case Compare Unit A or Compare Unit B. However, when using the register
or bit defines in a program, the precise form must be used, i.e., TCNT0 for accessing
Timer/Counter0 counter value and so on.
The definitions in
Table 15-1.
The Timer/Counter (TCNT0) and Output Compare Registers (OCR0A and OCR0B) are 8-bit
registers. Interrupt request (abbreviated to Int.Req. in the figure) signals are all visible in the
Timer Interrupt Flag Register (TIFR0). All interrupts are individually masked with the Timer
Interrupt Mask Register (TIMSK0). TIFR0 and TIMSK0 are not shown in the figure.
The Timer/Counter can be clocked internally, via the prescaler, or by an external clock source
on the T0 pin. The Clock Select logic block controls which clock source and edge the
Timer/Counter uses to increment (or decrement) its value. The Timer/Counter is inactive when
no clock source is selected. The output from the Clock Select logic is referred to as the timer
clock (clk
The double buffered Output Compare Registers (OCR0A and OCR0B) are compared with the
Timer/Counter value at all times. The result of the compare can be used by the Waveform
Generator to generate a PWM or variable frequency output on the Output Compare pins
(OC0A and OC0B).
pare match event will also set the Compare Flag (OCF0A or OCF0B) which can be used to
generate an Output Compare interrupt request.
The Timer/Counter can be clocked by an internal or an external clock source. The clock
source is selected by the Clock Select logic which is controlled by the Clock Select (CS02:0)
bits located in the Timer/Counter Control Register (TCCR0B). For details on clock sources
and prescaler, see
The main part of the 8-bit Timer/Counter is the programmable bi-directional counter unit.
ure 15-2
BOTTOM
MAX
TOP
shows a block diagram of the counter and its surroundings.
T0
).
The counter reaches the BOTTOM when it becomes 0x00.
The counter reaches its MAXimum when it becomes 0xFF (decimal 255).
The counter reaches the TOP when it becomes equal to the highest value in the count
sequence. The TOP value can be assigned to be the fixed value 0xFF (MAX) or the value
stored in the OCR0A Register. The assignment is dependent on the mode of operation.
Definitions
Table 15-1
“Timer/Counter0 and Timer/Counter1 Prescalers” on page
See “Using the Output Compare Unit” on page 121.
are also used extensively throughout the document.
for details. The com-
140.
9223B–AVR–09/11
Fig-